The quantitative study of how fluids (gases and liquids) move.

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Turbulent and smooth flow of water from a tap

When water flows normally from a tap, we can say it is turbulent ( Pic 1 ). But when we attach a piece of cloth to the opening of tap, water flow becomes smooth ( Pic 2 ). We can say that by touching ...
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movement of water in plants

Books say that water moves from places with a high water potential to places with a low water potential. The movement of water in plants are explained because water potential is lowest in leaves and ...
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Can water be made to flow through a pipe using only gravity and resulting in a higher elevation at the end of the pipe from its starting point?

I am not a physics guy, so be nice to the layman, guys! Please note: not a homework type problem. This is a scenario being considered for real-world application. I have not yet considered details ...
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Why can, or can not, a perfectly incompressible fluid exist?

Water is normally assumed to be an incompressible fluid - for example in the context of calculations involving water pressure. I wondered whether that is strictly true, or an approximation? Later I ...
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193 views

Problems implementing Lattice Boltzmann method

I want to make a basic 2D implementation of the Lattice Boltzmann method (in javascript) to simulate gas/fluid flow, but I have run into some problems and I can't seem to find the cause of it. ...
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How do cyclonic dust separators work? [closed]

I have a background in engineering so I am somewhat embarrassed to say that I don't understand how a seemingly simple cyclonic dust separator works. Here is a link to video of such a device in ...
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Sound in stirred hot fluids

If a hot beverage in a cup gets stirred, the sound of the spoon changes. You can easily hear this if you repeatedly cling the spoon to the cup ground after stirring. The cling sound will raise in tune ...
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Stokes law in 2-dimensions

Stokes' law states that force on slow moving sphere (i.e. $Re\ll1$) in liquid is $$ F_d = 6 \pi \mu R V $$ In two dimensions we are in trouble (flow around disk in 2d or around cylinder in 3d), ...
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Pressure as a scalar? [duplicate]

Why do treat pressure as a scalar? We know that $P=F/A$ (Pressure = Force/Area) and force is vector quantity, so then why should pressure not be a vector?
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Is it possible that any spatial variation in press disturbance without gravity?

When gravity (and buoyancy), acting as the only body force of a liquid, is ignored. Is it possible that any spatial variations in pressure perturbation are the result of fluid motion or some other ...
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174 views

Why velocity potential?

(Aero-)Acoustics (among other parts of fluid-dynamics) loves the velocity potential $\varphi$ defined as $\vec{v} = \nabla \ \varphi$ with the condition of $\nabla \times \ \vec{v} = 0$. I am ...
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Why is there a breakdown in Kolmogorov scaling in turbulence?

Why is there a breakdown of Kolmogorov scaling in turbulence? What causes intermittency?
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Centripetal forces acting on fluid flow in a bent pipe?

Yesterday I answered this question, using analysis of forces from this web page. But today I started having doubts regarding the completeness of this analysis. It occurred to me this approach does ...
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How to visualize that the critical flow conditions in a heat added flow remains constant?

In a variable area nozzle, where compressibility is observable, if the flow goes from point A to point B and heat addition is considered. How do we see that the critical speed of sound (imaginary ...
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39 views

Can we tune the exhaust system of a gasoline car to be as quiet as an electric car? [closed]

Electric cars are quiet, so quiet that legislation is proposed in many countries, to make them audible to pedestrians, for obvious safety reasons. My question: is it possible, without getting into ...
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340 views

Force applied to a 90 degree elbow by hydraulic flow

I have a question about forces applied to a 90 degree elbow. Essentially we have a 90 degree pipe bend used to redirect discharge from a water system, which can discharge high volumes and pressures ...
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Equating $\frac{dp}{dt}$ and $F$ at a hydraulic jump?

A journal I am reading about circular hydraulic jumps provides the equation and I quote: $H$= Depth after hydraulic jump, $h$= depth before hydraulic jump, $V$= velocity after hydraulic jump, $v$= ...
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152 views

Show that the boundary layers diffuse out from the plate with speed $\sqrt{\frac{\nu}{t}}$ [closed]

I was wondering if somebody would be able to help me with this problem. I know how to solve it using dimension arguments but I'm unsure what is meant by transformation techniques. Any help would be ...
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29 views

Help me in the measurement of small deformation of a gas-liquid interface

I am new to optical measurements and sorry for any stupid question? I am also not sure whether this is a right place to discuss the experimental issues... Recently, I am trying to measure the small ...
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105 views

Is the reason why water in a cup rises if a solid is placed within it the ideal gas law?

Let's say you have a cylinder with water that takes up volume V and has temperature T. When a solid mass is placed within the cylinder, the water momentarily takes up a smaller volume within the ...
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fluid dynamics of two streams in single pipe

Background: The fetus needs to deliver both high and low oxygen tension blood within the same pipe (inferior vena cava) back to the right atrium. Blood entering the right atrium is able to cross an ...
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Why are roofs blown away by wind?

Whenever there are high winds, such as in storms, thin metal roofs on sheds as well as concave roofs on huts are sometimes blown away. One explanation provided to me said that the higher velocity of ...
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148 views

Lift and drag coefficients on other planets

The question I'm trying to answer seemed simple: how hard would it be to fly on a planet with lower gravity but also thinner atmosphere compared to Earth. If the answer could hint me at how much ...
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How does a sponge “suck” up water against gravity?

If I take a sponge and place it in a shallow dish of water (i.e. water level is lower than height of sponge), it absorbs water until the sponge is wet, including a portion of the sponge above the ...
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can gasoline be compressed

Simple question, can gasoline be compressed? Is it compressible as in how water is incompressible. Would one be able to store x gallons of gasoline in a pressurized container that is x-y size. For ...
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How does a 1,000 mph car maintain vertical stability with tiny front wings?

Formula 1 cars have wings to help keep them on the road during cornering, they don't need them whilst moving in a straight line. And that's ok , they "only" travel at 220 mph. In a different speed ...
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71 views

Does charged saltwater repel itself?

if you take a volume of saltwater, and charge it to a high voltage, will the water molecules repel each other and climb the walls of the container slightly, or does the fact that water is polar ...
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135 views

How does a ball cause a splash? (With the relevant math)

Problem Statement: Imagine a spherical ball is dropped from a height $h$, into a liquid. What is the maximum average height of the displaced water? For instance, although one particular drop of water ...
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229 views

Understanding equation for force acting on stones skipping over water

I think I have a decent conceptual understanding of the forces at work when stones are skipped over water. My question pertains to this equation, $$ F = C_L\rho U^2S\sin({\alpha + \beta}) $$ which ...
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172 views

Forces of reaction and torque of water flowing out of a vessel

Consider the problem wherein we are told to find out the force of reaction of water flowing out of a vessel through an orifice. I attempt to compare it with a shotgun and a bullet as a conservation ...
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79 views

Perturbations on a fluid thread and hydrodynamic instability

I know that due to fluid instability there are some perturbations ( picture below ) after all of these, my question is the reason of existence of these perturbations. Why exactly they appear? Shear ...
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42 views

Calculating the 'splash zone' of a water droplet?

How would you calculate the probabilistic area for a water droplet splash zone? So far my guess would be to find out how much the wind is acting upon it horizontally, find out how fast its falling, ...
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138 views

Coffee Straw Physics

When I put my little, cylindrical coffee straw into my coffee, the liquid immediately rises about half a centimeter up the straw without provocation. This is also the amount of coffee that the surface ...
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Pressure - How much fluid can void support?

Please refer to the attached (terrible) drawing while reading the post ! After watching some videos about the U-tube effect in drilling, a question came to me. Imagine one fluid of heavy density ...
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growth rate of the instability of a laminar flow?

I was just studying something about The Plateau-Rayleigh Instability that I saw something called Growth rate of the instability. My question is the exact physical definition of growth rate of ...
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345 views

Name of experiment

I'm seeking the name of or reference for an experiment I once saw in a college physics class. At the beginning of one class the instructor repeatedly wound a wiper that spread a blot of some type of ...
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What is the reason behind Laminar Flow and the ability to return to near-original state in Laminar fluids?

Laminar flow here refers to the smooth disruption of fluid in layers, in a medium. A peculiar trait in a few examples of laminar flow is the ability to return to near-original state, as demonstrated ...
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What happened here in this Taylor-Couette Flow experiment?

I came across this video of Taylor-Couette Flow on YouTube. Originally I was looking for a visualization of the wavy Taylor vortices induced by the angular motion of the inner cylinder. However, I ...
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Maximum pressure in a die-less wire drawing apparatus

I ran across this patent: http://www.google.com/patents/US4549421 and was interested by the idea of reducing the cross sectional area of a wire with only shear stress caused by the wire being pulled ...
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Kernel normalization in Smoothed Particle Hydrodynamcs

I am trying to write some code for a fluid simulation. I have read about Smoothed Particle Hydrodynamics (SPH) and my question is related to the properties of the smoothing kernel. How do I set the ...
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What does air “feel” like to a flying mosquito in terms of viscosity?

If I go for a walk at, say 4 km/hour, unless there is a breeze blowing, I probably won't notice the air around me at all. If I go for a swim though, I will immediately notice the viscosity of the ...
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110 views

Why in fluid interface, liquids tend to reduce their surface?

I've read something about fluid interface and know that liquids tend to reduce their surface but my question is that why do they tend to reduce their surface?
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116 views

How to calculate maximum wind speed in a funnel (Venturi Effect)?

It must be in an open area, so nothing forces the wind, just himself. Lets assume that the wind is blowing only one direction. I assume that at some point the speed will stop increasing and it will ...
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What law of physics did these people think they disproved?

I was trying to understand the Physics of how water gets to the leaves of a tree and found this article which claims some experiment disproved a 300 year old law of Physics. Sounds pretty unlikely... ...
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Which travels further, a football or a soccer ball, when thrown?

I recently answered this question: Laminar Versus Turbulent Flow and it caused me to think of the following situation. Given that an (American) football weighs between Football 400 to 430 grams, and ...
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What is the difference between flow and expansion?

Fluids (both liquids and gases) will move from one point in space to another due to a potential gradient. Some examples may be: 1) horizontal pipe flow, a fluid will move from a region of high ...
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199 views

How do hydrofoils tack without a keel

I asked a while back about how sails produce a forward force on a boat and got some good answers and (I thought) a good understanding: essentially the sail produces a force that has some non-zero ...
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conservation of energy for a viscous fluid

I have a pair of questions about conservation of energy and momentum in a viscous fluid. First, energy. Symon's Mechanics states an equation for conservation of energy in a droplet of a non-viscous ...
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84 views

volumetric flow rate due to Bernoulli's principle

for calculating the volumetric flow rate of a liquid coming out of a tube, I know that If the pipe is too short, the Hagen–Poiseuille equation may result in unphysically high flow rates; the flow is ...
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130 views

Unsteady and incompressible flow

In scientific literature, we know that the condition for incompressible flow is that the particular derivative of the fluid density $\rho$ is zero: $$ \frac{D\rho}{Dt}=0$$ and for steady flows the ...